Electrophotographic copiers of the image-transfer type, or plain-paper copiers as they are generally called, are well known in the art. In copiers of this type, an electrostatic latent image is first formed on a photoconductor by uniformly charging the photoconductor and then exposing the photoconductor to a light image of an original document to discharge portions of the photoconductor in a pattern corresponding to the graphic matter on the original. The photoconductor bearing the latent image is then subjected to the action of a developer, or toner, to form a developed toner image. The toner image is then transferred to a carrier sheet such as paper. Generally, in electrophotographic copiers employing the process described above, the photoconductor comprises an endless member, usually in the form of a drum, that is continuously moved at a predetermined velocity throughout the entire copy cycle. To transfer the developed toner image from the photoconductor to the carrier sheet, the sheet is brought into close proximity or actual contact with the photoconductor, while moving at the same velocity, in a transfer station.
The velocity of the photoconductor, and of the carrier sheet during image transfer, is the product of the spacing between the leading edges of successive images on the photoconductor, which must be at least the length of a copy sheet, and the copy rate. For example, for a copy rate of 60 copies per minute and a spacing of 16 inches between successive leading edges of images the sheet velocity in the transfer station must be 16 inches per second. If the copier continues to feed the sheet at this velocity as it delivers the sheet to a stacking tray, stacking will occur in an uncontrolled manner, and the edges of the stacked sheets will generally be out of register with one another. It is known in the art, as shown in Lauren U.S. Pat. No. 3,942,786, Walkington U.S. Pat. No. 4,040,617 and Crawford U.S. Pat. No. 4,073,223, that the evenness of sheet stacking may be improved by decelerating sheets before they are delivered to a stacking tray. However, the existing mechanisms for achieving this deceleration are relatively complicated mechanically, and are not readily adaptable for use with copy sheets of varying lengths or spacings.